1. Field Of The Invention
The present invention relates to data processing of color image data, and particularly relates to data processing of color image data for print out by an ink jet printer such that true black ink is replaced by process black ink, so as to minimize bleeding between colored regions and black regions.
2. Description Of The Related Art
Conventional ink jet printers customarily obtain full color print out through the use of four different inks: three colored inks (such as cyan, magenta and yellow) and a black ink. At the border between a black area print out and a colored area print out, however, one known problem contributes to degraded print out quality: the black ink bleeds into the colored region, thereby degrading the quality of the colored area and blurring the border between the black area and the colored area.
The reason that the black ink bleeds into colored ink areas is that the black ink and the colored inks are of different types. Specifically, the black ink is an evaporation-type pigment ink whereas the colored inks are penetration-type inks. Accordingly, proposals to address the problem of black ink bleeding into colored areas have included a proposal to replace pigment black ink with process black for a few pixels at a black border area. More particularly, for a few black pixels adjacent the border between a black area and a colored area, the black pixels are printed by process black (i.e., superimposingly printing cyan, magenta and yellow), with the remainder of the black area being printed by pigment black ink. Because process black is formed using the same kind of ink as the colored regions, bleeding between the colored region and the black region at the border is minimized. In addition, any bleeding of the pigment black ink into the process black region is not visually detectable because the black ink bleeds into a near black-colored region.
Previous proposals for determining the location of black border regions, and for replacing black ink with process black, are described in Application Ser. No. 08/901,719, "Printer Driver For A Color Printer", and Ser. No. 08/972,308, "Printing With Multiple Different Black Inks", the contents of both of which are incorporated herein by reference as if set forth in full. As described in those applications, print data for a target pixel is examined in a 5.times.5 pixel grid surrounding the target pixel. If print data for the target pixel indicates that the target pixel is colored, then any black in all surrounding pixels in the 5.times.5 grid is replaced with process black so as to minimize the adverse effects of black ink bleeding.
While such techniques significantly enhance print quality, they are at the same time computationally-intensive since for every pixel of print information, a surrounding 5.times.5 grid of pixels must be examined. When black replacement is embodied in a print driver in a personal computer, powered for example by a Pentium.TM. processor, the number of comparisons needed to process each pixel of print data is so great that implementation of this technique is tediously slow for an average computer user. That is, each comparison, and resulting branch if a black ink replacement becomes necessary, requires a significant number of processor clock cycles. The problem is worsened in processors like the Pentium.TM. processor which employ dual processor pipelines together with branch prediction. For such processors, there are significant processor cycle penalties making unpredicted branches. See generally Schmit, "Pentium.TM. Processor Optimization Tools", Academic Press, Inc., 1995, pp. 101 to 192.
Accordingly, although black ink replacement with process black is a desirable feature, it is often omitted because of processor penalties associated with its implementation.